T3D A Dull Double

[Bruce Springsteen <bsspringsteen@xxxxxxxxx>]

Two obtuse questions, which I am  mentally blocked on.  None warrants
its own thread, though they may go their separate ways or just bounce
back to me off-list - I don't know.

1.  I've been told that measuring distance to near, far and other
points from the camera is best done from the film plane, at least when
the zoom on my SLR is involved.  But in the maofd general equation -
as I look at the illustration that goes with the article - "lens
center to near point" (an) and "lens center to far point" (af)
correspond to "lens center to film plane" (a') in the similar triangle
relationtionships T1:T2 and T3:T4 by which the formula is derived. 
Measuring subject distances from the film plane, instead of lens
center (or "nodal point") would seem to reduce a' to zero, making
triangles T1 and T3 non-existant and reducing deviations on the film
plane to zero.  What have I done to myself?  Is this a special case or
what? ;-) Obviously I am twisted in some fallacy resulting from my
profound mathematical ignorance.

2.  A Klooswyk mentions Ferwerda's list of various
infinity-in-projection philosophies, the two favorites being
(1)window-at-the-screen, and
(2)infinity-separation-at-standard-interpupillary.  But is there some
well-known calculation that tells how far to place the projector from
the screen to achieve (1) and (2) simultaneously?  And what image size
would typically result? I've never projected anything, and have almost
never seen projection, so my experience cannot help me, and I don't
see the answer where I have looked.

Thanks.  I look forward to contributing more interesting inquiries in
future, when the mental fog clears. :-)

Bruce (Rhetoric on Ice) Springsteen.

 



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